High plasma calcitonin levels increase electrolyte secretion via mediators such as prostaglandins, serotonin, or vasoactive intestinal peptide (VIP). However, recent studies on pathophysiologic mechanisms of diarrhea in MTC found normal levels of these mediators, normal small bowel absorption, and normal gastric and small bowel transit. This evidence concerns the gene VIP and medullary thyroid gland carcinoma.